Metals are understood to exhibit primarily nondirectional metallic bonds, which allow bonds to break under the application of a stress/load and then reform allowing metals the ability to have intrinsic ductility and the ability to deform plastically. Mechanistically, metals may deform at room temperature primarily through the movement of dislocations. Dislocations may be understood as one-dimensional type defects which can exhibit edge, screw, or mixed character and move by breaking the bonds of individual atoms one at a time resulting in a displacement of the atoms by one Burgers vector. Dislocations are found to move on their slip systems, which depending on the specific crystal structure and space group, may involve specific planes and specific crystallographic directions.
It may be appreciated that in ionically bonded ceramic materials, dislocations can also play a role in deformation. However, these classes of materials have bonds which may be directional and involve transfer of electrons and the formation of specific ions. Thus, after a particular bond is broken, this places positive ions next to positive ions or negative ions next to negative ions and the repulsion forces make it difficult to reform the bonds. Thus, due to the high strength of the ceramic bonds, ceramic materials can exhibit a relatively high hardness and strength which often are superior to that found in metals. However, ceramic materials may generally be brittle with an inherent inability to deform plastically.
Nanocrystalline metallic materials may also offer relatively high strength and hardness. Nanocrystalline materials may be understood to be, by definition, polycrystalline structures with a mean grain size below 100 nm. They have been the subject of widespread research since the mid-1980s when it was argued that metals and alloys, if made nanocrystalline, would have a number of appealing mechanical characteristics of potential significance for structural applications. But despite relatively attractive properties (high hardness, yield stress and fracture strength), it is well known that nanocrystalline materials may generally show a disappointing and very low tensile elongation and tend to fail in an extremely brittle manner. In fact, the decrease of ductility for decreasing grain sizes has been known for a long time as attested, for instance, by the empirical correlation between the work hardening exponent and the grain size as proposed for cold rolled and conventionally recrystallized mild steels. As the grain size is progressively decreased, the formation of dislocation pile-ups may become more difficult and their movement is quite limited by the large amount of 2-d defect phase and grain boundaries. Thus, with the development of nanocrystalline grains, the achievement of adequate ductility (>1%) has been a challenge.
Metallic glasses are a class of materials which may exhibit characteristics which are both metallic like since they contain non-directional metallic bonds, metallic luster, and significant electrical and thermal conductivity, and ceramic like since relatively high hardness is often obtained coupled with brittleness and the lack of tensile ductility. Amorphous metallic alloys (i.e., metallic glasses) represent a relatively young class of materials, having been first reported in 1960 when classic rapid-quenched experiments were performed on Au—Si alloys. Since that time, there has been remarkable progress in exploring glass forming alloy compositions, seeking elemental combinations with ever-lower critical cooling rates for the retention of an amorphous structure. Metallic glasses are understood to be supercooled liquids which may exist in solid form at room temperature but have structures which are relatively similar to what is found in the liquid with relatively short range order present. Metallic glasses may have free electrons, exhibit metallic luster, and exhibit metallic bonding similar to what is found in conventional metals. All metallic glasses may be considered metastable materials and when heated up, they will transform into crystalline state. The process is called crystallization or devitrification. Since diffusion is limited at room temperature, enough heat (i.e. Boltzman's Energy) needs to be applied to overcome the nucleation barrier to cause a solid-solid state transformation which is caused by glass devitrification. The devitrification temperature of metallic glasses can vary widely, commonly from 300 to 800° C. with enthalpies of crystallization commonly from −25 to −250 J/g. The devitrification process can occur in one or multiple stages. When occurring in multiple stages, a crystalline phase may be formed and then depending on the specific partition coefficient, atoms may either be attracted to the new crystallites or rejected into the remaining volume of the glass. This may result in a more stable glass chemistry which may necessitate additional heat input to cause partial or full devitrification. Thus, partially devitrified structures can result in crystalline precipitates in a glass matrix. Commonly, these precipitates may be in the size range of 30 to 125 nm. Full devitrification to a completely crystalline state may result from heat treating above the highest temperature glass peak which can be revealed through thermal analysis such as differential scanning calorimetry or differential thermal analysis.
Due to the extremely fine length scale of the structural order (i.e. molecular associations) and near defect free nature of the material (i.e. no 1-d dislocation or 2-d grain/phase boundary defects), relatively high strength (and correspondingly hardness) may be obtained which can be on the order of 33 to 45% of theoretical. However, due to the lack of crystallinity, dislocations may not be found and so far there is does not appear to be a mechanism for significant (i.e. >2%) tensile elongation. Metallic glasses may exhibit relatively limited fracture toughness associated with the rapid propagation of shear bands and/or cracks which may be a concern for the technological utilization of these materials. While these materials may show adequate ductility by testing in compression, when testing in tension they may exhibit elongations very close to zero and fracture in the brittle manner. The inherent inability of these classes of material to be able to deform in tension at room temperature may be a relatively limiting factor for potential structural applications where intrinsic ductility may be needed to avoid catastrophic failure.
Owing to strain softening and/or thermal softening, plastic deformation of metallic glasses may be relatively highly localized into shear bands, resulting in a relatively limited plastic strain (less than 2%) and catastrophic failure at room temperature. Different approaches have been applied to enhanced ductility of metallic glasses including: introducing heterogeneities such as micrometer-sized crystallites, nanometer-sized crystallites, glassy phase separation, or by introducing free volume in amorphous structure. The heterogeneous structure of these composites may act as an initiation site for the formation of shear bands and/or a barrier to the rapid propagation of shear bands, which may result in enhancement of global plasticity in compression and sometimes a corresponding decrease in the strength. Recently, a number of metallic glasses have been fabricated in which the plasticity was attributed to stress-induced nanocrystallization or a relatively high Poisson ratio. It should be noted, that with these approaches, metallic glasses may exhibit enhanced plasticity during compression tests (12-15%) but their tensile elongation may not exceed 2%. Very recent results on improvement of tensile ductility of metallic glasses was published when 13% tensile elongation was achieved in a zirconium based alloys with large dendrites (20-50 μm in size) embedded in glassy matrix. It should be noted that this material is primarily crystalline and might be considered as a microcrystalline alloy with residual amorphous phase along dendrite boundaries. The maximum strength of these alloys as reported is 1.5 GPa. Thus, while metallic glasses are known to exhibit favorable characteristics of relatively high strength and high elastic limit, their ability to deform in tension may be extremely limited which severely limits the industrial utilization of this class of materials.